The use of so-called mass spectrometers to analyse the chemical composition of different types of samples in particular has been known for a long time; these analyse the sample material in respect of the distribution of the atomic/molecular weights. In this context, use is often made of so-called time-of-flight mass spectrometers, in which the atoms/molecules of the material to be analysed is ionized at first and then accelerated with the aid of electric fields, with a predetermined amount of kinetic energy being imparted on the atoms/molecules. Here the time of flight, which is the time the ions require to reach a detector from the point at which they are ionized, is measured, with the ionization possibly not being brought about continuously, but in a pulsed fashion, for example with the aid of laser pulses such that there is a defined start time for the time-of-flight measurement.
The time of flight required by the ionized atoms/molecules for the predetermined path length is a measure for the mass thereof because, at a fixed prescribed kinetic energy, those atoms/molecules with a great mass will require a longer period of time than light ones for covering the path length.
In order now to increase further the mass resolution of such a time-of-flight mass spectrometer, it was found to be advantageous to insert a so-called reflector along the path length covered by the ionized atoms/molecules. Here, the accelerated, ionized atoms/molecules (ions) firstly move towards the reflector, are decelerated therein and are then accelerated out of the reflector again in the opposite movement direction and in the direction of the detector. The reflector operates using an electrostatic field that has the same polarity as the charge state of the flowing-in ions.
In this context, work was firstly undertaken with a so-called net reflector, in which the electrodes are embodied in a net-like shape, which, inter alia, is connected with the advantage that the electrostatic field of the reflector does not extend beyond the volume thereof into the drift path along which the ions move to the detector. However, such net electrodes are connected with the disadvantage that some of the ions passing through the reflector are scattered by the net electrodes or are deflected by the electrostatic near fields of the nets, and thus no longer reach the detector. This in turn results in a decrease in the detection probability.
It is for this reason that use is made of reflectors with net-less ring electrodes, in which the electrodes are arranged along a common axis. This prevents the ions from being able to contact the electrodes in the reflector, and so, compared to net electrodes, this leads to increased transmission and hence an increased detection probability. DE 35 24 536 A1 has disclosed a time-of-flight mass spectrometer with such a reflector.
However, a disadvantage of such reflectors is that the drift paths, along which the ions move, in such a ring electrode arrangement are not free from field gradients. However, it is precisely assumed that ions with the same atomic/molecular weight are in a field-gradient-free region outside of the reflector and therefore have a constant speed. An edge field, which extends out of the reflector and into the actually field-gradient-free space results in a reduction in the mass resolution of the time-of-flight mass spectrometer.
In the case of reflectors with net-free electrodes, the penetration of the field within the reflector into the per se field-gradient-free drift path can in principle also be minimized by virtue of the fact that the diameter of the entry opening or the first ring electrode is kept very small. However, this reduces the range of the acceptance angle for the ions to be detected and in turn results in the number of ions entering the reflector, and hence, overall, the detection probability, being reduced due to higher transmission losses.